The objective of these studies is to determine the cause of ovarian quiescence observed during lactation in the rhesus monkey. FSH, LH, progesterone and estradiol have been measured by radioimmunoassay in plasma samples collected daily during the menstrual cycle of 5 rhesus monkeys. The level of these hormones will also be measured during lactation amenorrhea and compared to the levels observed during the menstrual cycle. The aim is to determine if the ovaries are quiescent during lactation because gonadotropins are low or if, on the other hand, the ovary is refractory to gonadotropic hormone stimulation.